Structure for offshore exploitation

Abstract

A structure for use in exploiting arctic offshore areas where floating ice masses may be present. The structure is comprised of a floating hull having ice-breaking capabilities which is moored by a plurality of mooring means which extend vertically from a moonpool in the hull to the marine bottom directly under the hull. The mooring means comprises flexible lines, e.g. nylon, steel, or the like or rigid conduits, e.g. drill pipe. Means are provided within the moonpool for tensioning the mooring means to thereby draw the hull downward to a position below its normal buoyant position thereby substantially eliminating vertical heaving of the hull. When an ice mass contacts the hull, tension on the mooring means is relaxed to thereby allow the hull to rock upward against the ice thereby generating the forces necessary for the ice-breaking operation. Due to the present mooring means, the hull moves only a relatively short lateral distance in breaking an approaching ice mass. This is important in floating drilling operations. Further, if repair or replacement of a mooring means is required, this can be accomplished even when ice surrounds the hull since the mooring means are anchored directly below the hull and are easily accessible through the moonpool.

What is claimed is: 
     
       1. An offshore structure adapted for operating in a body of water containing floating ice masses at least part of the year, said structure comprising: a floating hull having downwardly and inwardly sloping sides adapted for breaking said ice masses when contacted thereby;   a conduit extending vertically and centrally through said hull;   mooring means connected to said hull and extending substantially vertically from said conduit to the marine bottom underlying said hull;   means for anchoring said mooring means to said marine bottom directly below said hull; and   means on said hull for tensioning said mooring means and maintaining said hull downwardly in said body of water at a position below said hull's normal buoyant position when ice mass is not of concern, and for relaxing tension on said mooring means when desirable to rock said hull against an ice mass.   
     
     
       2. The offshore structure of claim 1 wherein said mooring means comprises: a plurality of substantially elastic, nylon lines each of which is capable of stretching when a predetermined load is applied thereto.   
     
     
       3. The offshore structure of claim 2 wherein said tensioning means comprises a plurality of individual tensioning means, one for each of said plurality of nylon lines, each of said individual tensioning means comprising: a payout spool onto which one end of said each nylon line is stored; and   a takeup spool onto which the other end of said each nylon line is stored;   and wherein said means for anchoring said mooring means to said marine bottom comprises:   a base on said marine bottom; and an individual sheeve mounted on said base for each of said plurality of nylon lines over which said other end of said each nylon line is passed before said other end is stored on said takeup spool.   
     
     
       4. The offshore structure of claim 3 wherein said tensioning means further comprises: a pair of spaced pulleys mounted on said hull over which said other end of said each nylon line is threaded before said other end is stored on said takeup spool; and means for driving one of said pairs of pulleys.   
     
     
       5. The structure of claim 1 wherein said mooring means comprises: a plurality of steel cables.   
     
     
       6. The structure of claim 5 wherein said tensioning means comprises a plurality of individual tensioning means one for each of said plurality of steel cables, each individual tensioning means comprising: a first support fixed to said hull with an opening through which said each steel cable passes;   a second support slidably mounted on said hull above said second support;   means for attaching said each steel cable to said second support; and   means for moving said second support relative to said first support.   
     
     
       7. The offshore structure of claim 6 wherein said means for attaching said each steel cable to said second support comprises: collapsible means positioned on said second support over an opening in said second support through which said each steel cable passes, said collapsible means adapted to fail upon the application of a predetermined load thereon, and   means for securing said each steel cable to said collapsible means whereby upon application of said predetermined load, said collapsible means collapses to allowed limit movement between said each steel cable and said second support.   
     
     
       8. The offshore structure of claim 1 including: a conduit passing vertically through the center of said hull and affixed thereto to thereby form an opening therethrough;   and wherein:   said means for tensioning said mooring means is affixed in said conduit.   
     
     
       9. The offshore structure of claim 1 wherein said mooring means comprises: a plurality of rigid conduits.   
     
     
       10. A method of using an offshore structure in a body of water containing ice masses at least part of the year said offshore structure comprising a floating hull having downwardly and inwardly sloping sides adapted for breaking ice masses when contacted thereby said method comprising: anchoring said structure to the marine bottom with mooring means directly below said hull;   maintaining said hull downwardly in said body of water at a position below the normal buoyant position of said hull when ice masses are not of concern; and   relaxing tension on said mooring means when desirable to rock said hull against an ice mass.